1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor, which detects torque based on a change in a magnetic property due to magnetostriction, and to an electric power steering apparatus including such a magnetostrictive torque sensor therein.
2. Description of the Related Art
One known type of contactless torque sensor is a magnetostrictive torque sensor, which detects torque based on a change in a magnetic property due to magnetostriction. Such a magnetostrictive torque sensor is used to detect the steering torque of an automotive steering apparatus (see Japanese Laid-Open Patent Publication No. 2004-309184).
A magnetostrictive torque sensor of the type described above comprises two magnetostrictive films (a first magnetostrictive film and a second magnetostrictive film) having different magnetic anisotropic properties and which are mounted on a rotatable shaft, and first and second detecting coils disposed in confronting relation to the first and second magnetostrictive films respectively. When a torque is applied to the rotatable shaft in order to twist the rotatable shaft, magnetic permeabilities of the first and second magnetostrictive films change, thereby changing the inductances of the first and second detecting coils. Therefore, torque can be detected as a result of changes in the impedances or induced voltages of the first and second detecting coils.
As described above, operation of the magnetostrictive torque sensor relies on a phenomenon in which, when ends of the rotatable shaft are twisted in opposite directions in order to apply a torsional torque to the first and second magnetostrictive films, magnetic properties such as the magnetic permeability or loss of the first and second magnetostrictive films change due to a magnetostriction effect (also referred to as an “inverse magnetostriction effect”).
Since the rotatable shaft is required to be mechanically twisted and have bending strength, the rotatable shaft is made of a base material composed of an iron alloy, which is required to be subjected to a heat treatment, such as thermal refining, surface treatment, surface quenching, dip quenching, etc. However, such a heat treatment poses problems in that a significant amount of carbon remains in the base material of the rotatable shaft, thus making the rotatable shaft easily magnetizable.
As described above, since the rotatable shaft can be magnetized easily, if a magnetostrictive torque sensor with the rotatable shaft is incorporated in an electric power steering apparatus, then alternating magnetic fluxes due to an alternating magnetic field from an electric motor on an electric vehicle or the like tend to pass through the rotatable shaft, which is used as a steering shaft. As a result, the magnetostrictive torque sensor outputs a sensor signal, which includes noise generated by such alternating magnetic fluxes. Such noise produces a vibratory sound, which adversely affects the sensor housing as well as the acoustic plate and resonant plates of other devices, which tend to generate noise (alternating magnetic flux sounds). Although such noise can be removed by a low-pass filter, the low-noise filter causes signal delay, which lowers the response of the detected signal, and hence the magnetostrictive torque sensor is unable to generate an output signal having a wide dynamic range.
Such an electric power steering apparatus, which incorporates the magnetostrictive torque sensor therein, fails to give the driver a smooth steering feel, because the electric motor of the electric power steering apparatus that assists the driver in turning the steering wheel suffers from a large delay during operation thereof.